Recently, it has become increasingly desirable to make devices having semiconductor integrated circuits small in size, and flat packages have become frequently used in such integrated circuits.
As a lead frame for a flat package, for example, a construction as shown in FIG. 2 is known. In particular, the lead frame (3) which is to be provided with terminal parts on all four edges thereof needs to have metal-coated parts for improving the weldability, for example, as coated with a metal of Ag-brazing material, Al, Cu or the like, on the parts to be wire-bonded.
For providing such metal-coated parts, heretofore a lead frame as worked has been subjected to (a) a vapour-deposition method or (b) a plating method. However, a method having a higher producibility is desired because the noted methods have some drawbacks. In particular, in accordance with the vapour-deposition method (a), not only expensive plants are necessary but also the parts which are not to be coated with metals need to be masked. Therefore, the vapour-deposition speed is slow and the producibility is low. On the other hand, in accordance with the plating method (b), metals usable in the method are limited as some metals cannot be plated, and the thickness of the metal layer to be efficiently plated is also limited.
In order to previously provide the metal-coated parts, there is a method of spotwise applying the metal-coated parts (2) onto the lead frame material (1) with high accuracy, as shown in FIG. 1, followed by press-working or etching the spotwise metal-coated material.
For instance, a technique in which an Al or Ag foil is stripewise provided on a lead frame-forming strip material by pressure-welding or rolling and thereafter the unnecessary parts are removed mechanically or chemically so as to form the desired metal foil-coated spots on the strip material has been proposed (Japanese Patent Publication No. 59-1078). However, in accordance with the proposed technique, the yield of the coated metal is poor, the working efficiency is low as the necessary parts to be metal-coated must be masked when removing the unnecessary parts, and provision of the desired metal foil-coated spots, each having the desired size, at a desired pitch with high accuracy is difficult.
Although it is possible to lap metal foil pieces, each having a determined size, over a substrate material having a determined extremely short length at the determined positions to form a spotwise partial clad material having a determined length by pressure-welding or rolling, it is impossible to cut a metal foil to be coated into pieces, each having a determined size and a determined length, with high accuracy so as to attach the thus-cut metal foil pieces to a continuous long substrate material strip as unwound from a coil of the strip, with highly accurately adjusting the positions of the substrate material strip to be coated with the metal foil pieces, thereby to continuously tack the pieces on the strip at a determined pitch. For this reason, it is said that industrial mass-production of a spotwise partial clad material is impossible.
On the other hand, formation of a lead frame by tacking Al foil pieces on a lead frame material by spot-welding followed by cladding the pieces-tacked material has been proposed (Japanese Patent Publication No. 60-227456). However, the method of highly accurately adjusting the positions of the lead frame material to be coated with the metal pieces at a determined pitch for continuously tacking the pieces on the material is not concretely illustrated-, and therefore the proposed technique is not adaptable to practical use.
Additionally, the proposed technique would have another problem on the rollability of the Al-coated material, since the Al foil pieces as tacked by mere spot-welding are unmovably fixed at the welded parts.
Furthermore, the proposed technique would have still another problem in that the necessary pitch accuracy and dimension accuracy could not be achieved, since the pitch would fluctuate even by extremely slight variation of the rolling pressure in pressure-welding and rolling.
In view of the current situations mentioned above, the object of the present invention is to provide a method of manufacturing a spotwise partial clad material where metal foil pieces, each having a determined size, are clad on a substrate material at a determined pitch with high accuracy.
Another object of the present invention is to provide a method of manufacturing a spotwise partial clad material where a metal foil strip is cut into pieces having a determined size and a determined length with high accuracy, the pieces are continuously tacked onto a lead frame material by accurately adjusting the positions to be tacked, and thereafter the material as tacked with the metal foil pieces is pressure-welded or rolled to form a metal foil pieces-clad material, whereupon the pitch of the metal foil pieces as clad on the substrate material by rolling is highly accurate.
Still another object of the present invention is to provide a method of manufacturing a spotwise partial clad material where a metal foil strip is continuously tacked onto a lead frame material by accurately adjusting the positions to be tacked, the unnecessary parts of the tacked metal foil strip are cut and removed to thereby spotwise arrange the metal foil strip on the substrate material before rolling, and thereafter the metal foil pieces-tacked are pressure-welded and rolled to finally efficiently give a metal foil pieces-clad material, whereupon the metal foil pieces are clad on the material with high accuracy after rolling.